To drill a well, a drill bit bores thousands of feet into the crust of the earth. The drill bit typically extends downward from a drilling platform on a string of pipe, commonly referred to as a “drill string.” The drill string may be jointed pipe or coiled tubing, through which drilling fluid is pumped to cool and lubricate the bit and lift the drill cuttings to the surface. At the lower, or distal, end of the drill string is a bottom hole assembly (BHA), which includes, among other components, the drill bit.
To obtain measurements and information from the downhole environment while drilling, the BHA includes electronic instrumentation. Various tools on the drill string, such as logging-while-drilling (LWD) tools and measurement-while-drilling (MWD) tools, incorporate the instrumentation. Such tools on the drill string contain various electronic components incorporated as part of the BHA that generally consist of computer chips, circuit boards, processors, data storage, power converters, and the like.
Downhole tools must be able to operate near the surface of the earth as well as many hundreds of meters below the surface. Environmental temperatures tend to increase with depth during the drilling of the well. As the depth increases, the tools are subjected to a severe operating environment. For example, downhole temperatures are generally high and may even exceed 200° C. In addition, pressures may exceed 138 MPa. There is also vibration and shock stress associated with operating in the downhole environment, particularly during drilling operations.
The electronic components in the downhole tools also internally generate heat. For example, a typical wireline tool may dissipate over 135 watts of power, and a typical downhole tool on a drill string may dissipate over 10 watts of power. While performing drilling operations, the tools on the drill string also typically remain in the downhole environment for periods of several weeks. In other downhole applications, drill string electronics may remain downhole for as short as several hours to as long as one year. For example, to obtain downhole measurements, tools are lowered into the well on a wireline or a cable. These tools are commonly referred to as “wireline tools.” However, unlike in drilling applications, wireline tools generally remain in the downhole environment for less than twenty-four hours.
A problem with downhole tools is that when downhole temperatures exceed the temperature of the electronic components, the heat cannot dissipate into the environment. The heat may accumulate internally within the electronic components and this may result in a degradation of the operating characteristics of the component or may result in a failure. Thus, two general heat sources must be accounted for in downhole tools, the heat incident from the surrounding downhole environment and the heat generated by the tool components, e.g., the tool's electronics components.
While the temperatures of the downhole environment may exceed 200° C., the electronic components are often rated to operate at no more than 125° C. Thus, exposure of the tool to elevated temperatures of the downhole environment and the heat dissipated by the components may result in the degradation of the thermal failure of those components. Generally, thermally induced failure has at least two modes. First, the thermal stress on the components degrades their useful lifetime. Second, at some temperature, the electronics may fail and the components may stop operating. Thermal failure may result in cost not only due to the replacement costs of the failed electronic components, but also because electronic component failure interrupts downhole activities. Trips into the borehole also use costly rig time.
There are at least two methods for managing the temperature of thermal components in a downhole tool. One method is a heat storing temperature management system. Heat storing temperature management involves removing heat from the thermal component and storing the heat in another element of the heat storing temperature management system, such as a heat sink. Another method is a heat exhausting temperature management system. Heat exhausting temperature management involves removing heat from the thermal component and transferring the heat to the environment outside the heat exhausting temperature management system. The heat may be transferred to the drill string or to the drilling fluid inside or outside the drill string.